1. Field of the Invention
The present invention relates to a stainless steel separator for such a fuel cell as a solid polymer-type, which is driven at a low temperature.
2. Description of the Related Art
A solid polymer-type fuel cell has the advantage that it is started in a short time and driven at a temperature lower than 100° C., in comparison with other types of fuel cells. Since it is built up by all solid members with simplified structure, it is maintained with ease and suitable for use in an environment subjected to vibrations or shocks. Moreover, it can be designed to small size due to high power density, and a fuel is efficiently consumed for power generation with less noise. Accounting these advantages, applicability of the solid polymer-type fuel cell to a power source of an automobile has been researched and examined in these days. Provided that a fuel cell, which gains the same mileage as a gasoline engine, is offered, an automobile can be driven under very clean conditions without generation of NOX and SOX. Discharge of CO2 can be also remarkably reduced.
A solid polymer-type fuel cell involves a solid macromolecular membrane which includes a proton-exchange group in its molecule and acts as a proton-transferring electrolyte. This type fuel cell is driven by the same way as other types of fuel cells, i.e. supply of a fuel gas such as hydrogen to one side of the membrane while supply of an oxidizing gas such as oxygen to the other side of the membrane.
A representative solid polymer-type fuel cell is built up by bonding graphite electrodes, i.e., an oxidizing electrode 2 (cathode) and a fuel electrode 3 (anode), respectively to both surfaces of a solid macromolecular membrane 1, and locating the membrane 1 together with gaskets 4, 4 between separators 5 and 5, as shown in FIG. 1A. The separator 5 faced to the oxidizing electrode 2 has an oxygen-supply opening 6 and an oxygen-discharge opening 7 formed therein, while the separator 5 faced to the fuel electrode 3 has a hydrogen-supply opening 8 and a hydrogen-discharge opening 9 formed therein. Air may be supplied through the opening 6 to the oxidizing electrode 2, instead of oxygen.
A plurality of grooves 10, which extends along flow directions of hydrogen (g) and oxygen (o), are formed in the separators 5, 5 in order to ensure sufficient supply and uniform distribution of hydrogen (g) and oxygen (o). Water-cooling means, whereby coolant water is supplied from openings 11, circulated in the separators 5, 5 and then drained from openings 12, are also built in the separators 5, 5 in order to release a heat during power generation.
Hydrogen (g) is fed from the opening 8 to a space between the fuel electrode 3 and the separator 5. Hydrogen (g) becomes a proton after discharge of an electron. The proton transfers through the membrane 1 and accepts an electron at the oxidizing electrode 2. Thereafter, hydrogen is burnt with oxygen (o) or air fed to a space between the oxidizing electrode 2 and the separator 5. Electric power is outputted by connecting a load resistor between the oxidizing electrode 2 and the fuel electrode 3.
Since electric power generated by one fuel cell is very tiny, a plurality of cells each composed of the membrane 1 sandwiched between the separators 5, 5 are stacked together, as shown in FIG. 1B, in order to accumulate electric power to a level suitable for practical use. However, power-generating efficiency is substantially varied in accordance with electric resistance concerning contact of the separators 5, 5 with the graphite electrodes 2,3 as well as bulk resistance of the separators 5, 5 in the stacked assembly. Accumulation of power-generating efficiency needs separator material good of electric conductivity with small contact-resistance in contact with a graphite electrode. In this sense, a graphite separator has been used so far in the same way as in a phosphate-type fuel cell.
A graphite separator is manufactured by cutting a graphite block to a predetermined shape and machining the shaped block for formation of various openings and grooves. Due to the manufacturing process, a large sum of expenses is inevitably required for material and processing. As a result, a fuel cell becomes very expensive in total, and productivity is also inferior. Moreover, a separator made of brittle graphite is easily damaged by vibrations or shocks. These disadvantages are eliminated by use of a metal separator instead of a graphite separator. The metal separator is manufactured by punching or pressing a metal sheet, as disclosed in JP 8-180883 A.
A metal separator is fairly superior of various properties, e.g. shock-resistance gas-impermeability and strength, compared with a carbon separator. But, prior art merely suggest applicability of the metal separator to a solid polymer-type fuel cell, but do not actually propose the metal material, which is good of electric conductivity and corrosion-resistance necessary for the purpose.
Stainless steel is a representative material resistant to a strong acid. Its acid-resistance is derived from a tough passive film formed on its surface, but the passive film causes increase of surface or contact-resistance. As increase of contact-resistance, a large amount of Joule heat is generated in the contact area. Consequently, an electric energy is consumed as a heat loss, and power-generating efficiency of a fuel cell is significantly reduced. Formation of a coating layer excellent in both electric conductivity and corrosion-resistance is one idea for improvement of electric conductivity of a metal separator. In this concern, the metal separator proposed by JP 8-180883 A is coated with a noble metal. However, the coating with an expensive noble metal raises a cost of a fuel cell and puts economical restrictions on development of a fuel cell. There is another idea for reduction of contact-resistance by formation of a carbonaceous coating layer, but a manufacturing process is complicated and expensive.